A common method of collecting fibrous material, particularly fibrous mineral material emanating from spinners, involves distributing the fibers onto a collecting surface to form a pack. The fibers can be collected as a pack on a forming conveyor positioned within a forming hood beneath the sources of fibers, and the pack can be built up continuously on the moving conveyor. Such fiber collection apparatus is usually combined with an exhaust fan positioned beneath the forming conveyor to create suction through the forming conveyor and thereby force the fibers toward the conveyor to form the layer of fibers. Insufficient suction enables some of the fibers to circulate within the forming hood in eddy currents, known as "blowback." Apparatus for forming insulation packs as thick as three inches or more typically use eight or ten sources of mineral fibers, such as fiber-forming spinners, to successively deposit the mineral fibers onto the forming conveyors. The exhaust fans associated with such apparatus for producing relatively thick insulation packs must exert extremely strong suction on the mineral fibers in order to prevent blowback of the fibers emanating from the last spinners in the multi-spinner machine.
A common problem with such apparatus is that the suction pulls the initially deposited fibers onto the belt so forcefully that the bottom portion of the layer of fibers is crushed and ends up being more dense than the top portion. The greater the final thickness of the insulation pack, the greater the density variation from the bottom to the top of the pack. For example, in an R-19 insulation pack having a nominal density of 0.6 pounds per cubic foot (PCF), the top portion of the pack can have an average density of 0.4 PCF while the bottom portion of the pack can have an average density of 0.8 PCF or greater. As more and more fibers are deposited on the forming conveyor, greater and greater pressure is exerted on the fibers in the bottom portion of the layer. The force resulting from suction on the bottom fibers is increased by the drag force of the exhaust air on the upper fibers. Strong suction from the exhaust fan causes the fibers to be held flat against the fiber-forming conveyor, and any loose fibers are forced to fill voids at the very bottom of the fibrous layer.
The gradient of forces experienced by the fibers in the collection process results in a vastly different surface between the top and the bottom portions of the layer of fibers. Whereas any large holes in the bottom surface of the layer are filled so that only small spaces between fibers on the bottom surface are present, the top surface of the layer contains many large holes and voids. This nonuniformity in the surface of the layer of fibers is an undesirable characteristic. The large number of voids and large holes on the top surface of the layer of fibers is undesirable from the standpoint of feel and appearance in the final insulation product. The bottom surface of the insulation layer, however, would provide an excellent top surface for an insulation pack, since the bottom surface has no large holes or voids. There is a need in the manufacture of insulation packs for producing an insulation pack having as its top surface a smooth surface with uniformly deposited fibers, such as the surface produced at the bottom of an insulation layer.